Dark afternoons and wintry nights have seen the reappearance of the “one eyed monster” – vehicles with only one headlight operating.

It’s not just commercial vehicles that suffer from this problem, though they do seem to suffer more than private cars – perhaps because they have more than one driver and nobody takes responsibility for checking the overall condition of the vehicle, or simply they are in use more.

Having a headlight out is dangerous in two ways: not only can the driver of the vehicle not see properly in an unlit road with only half the usual light available, but the defective headlight means that other road users will have difficulty spotting the vehicle properly (or in some cases, knowing even what it is).

In a rear mirror, the “one eyed monster” might be mistaken for a motorbike. And at an urban “pinch point”, where you are trying to negotiate parked vehicles, it is difficult to gauge the width of the oncoming car with one headlight not working, increasing the possibility of a low speed “scrape”.

Where speed picks up, a misjudged overtake is more likely to result in a more serious collision if you cannot see the overtaking vehicle early enough, because it has no headlight on one side – especially if the offside (right) headlight is the one that does not work.

Defective headlights are not the only problem of course: lights not functioning properly at the back of a vehicle raise different problems. A brake light not working will make a rear end shunt more likely as the driver following takes longer to realise that the car ahead is stopping.

It is an offence to drive a vehicle with defective lighting; the lighting section of the MoT covers all exterior lamps required by the vehicle lighting regulations.

Nine times out of ten, defective lighting is simply a matter of replacing a blown bulb.

Yet checking your bulbs are still OK takes only a moment. In slow moving traffic, you can see if you have both headlights working by studying your reflection off the car in front.

And if when you park, you are by a window (reversing onto a drive, perhaps) you can use your mirrors to check that the brake light and red tail lights are all working properly. If in doubt, have somebody watch as you test them.

This article has been reproduced with the permission of the IAM (Institute of Advanced Motorists)